Microelectronic devices are typically fabricated of thin films of single element semiconductor materials such as silicon and germanium, or of multi-element semiconductor materials such as gallium arsenide. These semiconductor materials exist in three forms: monocrystalline, polycrystalline and amorphous. In monocrystalline structures (also referred to as "crystalline", "single crystal" or "c-" structures) the atoms are periodically arranged throughout the entire structure. In polycrystalline ("poly" or "pc-") structures, the solid is composed of many small single crystal regions. Amorphous ("a-") materials are devoid of any long range periodic structure.
In polycrystalline material, the basic periodicity of the crystal is found within any single crystal region. The single crystal regions, typically on the order of 1000 .ANG. in size, are referred to as grains, and are separated from one another by grain boundaries. The grain boundaries are composed of disordered atoms, and contain large numbers of defects due to incomplete bonding, often referred to as "dangling bonds". Intergranular spaces, devoid of any material may also be present. Thus, while the material inside the grains behaves similarly to crystalline material, the defects and departures from periodicity at the grain boundaries substantially alter the behavior of the poly material.
Polycrystalline semiconductor materials have many important applications in microelectronic device technology. For example, heavily doped polysilicon films have been widely used as gate electrodes and interconnections in metal oxide semiconductor (MOS) field effect transistor (FET) devices. Heavily doped polysilicon devices have also been used to form emitter structures in advanced bipolar technologies. Lightly doped polysilicon films have also been used as high valued load resistors in static memories and to refill trenches employed to isolate one device from another in very large scale integration (VLSI) devices. In these applications it is known to form a thin silicon dioxide (SiO.sub.2) layer on the polycrystalline layer to insulate the polycrystalline layer from an overlying layer.
The use of polycrystalline materials has been limited, however, because of the undesirable properties created by the grain boundaries and intergranular spaces which are inherent to the polycrystalline structure. In particular, the grain boundaries and intergranular spaces substantially alter the electronic properties of the material, serving as barriers and scattering centers of carriers such as electrons.
One class of microelectronic devices in which polycrystalline material has not heretofore been used is the class of ultra-thin multilayer devices including quantum well devices and superlattices. A quantum well device is a double barrier structure having a thin (on the order of tens of .ANG.ngstroms) well usually formed of low bandgap material, sandwiched between barrier layers usually formed of high bandgap material, to create a band-edge offset therebetween. Resonant tunneling in quantum well devices was observed as early as 1974 by L. L. Chang, L. Esaki and R. Tsu, in an article entitled "Resonant Tunneling in Semiconductor Double Barriers" (Applied Physics Letters, Vol. 24, No. 12, June 15, 1974). Quantum well devices have been extensively used for base and emitter regions of bipolar transistors and for field effect transistor gate regions.
A superlattice is a multilayer thin film structure consisting of multiple alternating layers of high and low bandgap material. Superlattices have been studied since as early as 1970 in an article entitled "Superlattice and Negative Differential Conductivity in Semiconductors" by L. Esaki and R. Tsu (IBM Journal of Research and Development, Vol. 14, page 61, January, 1970). Resonant tunneling in multibarrier structures has been studied since 1973 in an article entitled "Tunneling in a Finite Superlattice" by R. Tsu and L. Esaki (Applied Physics Letters Vol. 22 No. II, June 1, 1973). Superlattice structures are presently being used or under investigation for lasers, optical modulators, radiation hardened devices and other applications. Both quantum wells and superlattices rely on the phenomena of tunneling, in which electrons are trapped in a narrow, deep well which results from the thin layer being sandwiched between two barrier layers.
The materials suitable for use in quantum well/superlattice devices have heretofore been severely limited. Available materials have been limited because the well layer must be formed of monocrystalline material, in order to maintain tunneling. Tunneling is a manifestation of the wave nature of the electron, which is capable of transmission and reflection from a barrier. Electrons maintain coherence if the length of the well is less than the mean free path of the electron waves. In other words, if random scattering events occur in the well, thereby destroying the predictability of the electron waves within the well, the electron waves lose their coherence. Due to their regular molecular structure crystalline materials have no or few scattering events. Therefore, crystalline structures must be used to form the well portion of a quantum well. It is well accepted that polycrystalline materials have many scattering events, due to their irregular nature at the grain boundaries, thereby resulting in the loss of electron wave coherence. Consequently, polycrystalline materials have not been used for the well portion of quantum well structure.
The need to use monocrystalline materials in the well layer severely limits the available materials because the well layer must be formed on one of the barrier layers. It is well known in the semiconductor art that a monocrystalline layer can only be formed upon an underlying monocrystalline layer, in a process known as epitaxy. In a quantum well device, the underlying barrier layer must therefore be a monocrystalline layer. Moreover, not just any monocrystalline material may be employed. A high degree of lattice match between the barrier and well layers is required to produce a high quality epitaxial layer. For example, an excellent barrier on silicon is amorphous silicon dioxide. However, it is not possible to grow a silicon layer epitaxially on the amorphous layer in order to build a quantum well.